This invention relates to an impregnation resin composition, and in particular, to an impregnation resin composition which is suited for use in an insulation coil of a revolving electric equipment for vehicle or general industry, and of a stationary induction electric apparatus such as a transformer.
With the current trend to further miniaturize a revolving electric equipment for vehicle or general industry, an insulation coil of the revolving electric equipment is increasingly required to be excellent in insulating properties.
The insulating layer for such an insulation coil has been manufactured as follows. Namely, a woven fabric or a nonwoven fabric consisting of an inorganic fiber such as a glass fiber and polyamide fiber, or of an organic fiber; or an organic polymer film is employed as a base material. Then, a laminated mica is superimposed on the base material to obtain a mica sheet, which is then employed, together with a binder and a mica tape made from the mica sheet, to cover the surface of the coil conductor thereby to form a covering layer of desired thickness. This covering layer is then impregnated in a vacuum or under a pressurized condition with a thermosetting impregnation varnish of low viscosity, such as unsaturated polyester, epoxy resin or silicone resin, the impregnated varnish being subsequently cured to obtain an insulating layer.
As for this impregnation varnish, epoxy-based resins are generally employed because they are well-balanced in various properties. On the other hand, as for the binder for the mica insulating tape, a material which is not so sticky or a solid material is preferred in view of workability. It has been proposed, for the purpose of improving the heat resistance of the insulating layer, to employ, as an insulating tape for winding around the coil conductor, a mica insulating tape comprising a mixture of a solid epoxy resin compound exhibiting a high heat resistance, maleimide and a binder, or a mixture of epoxy resin, maleimide and a binder, and to employ an epoxy/acid anhydride-curing type varnish for the impregnation of this mica insulating tape.
When an acid anhydride is employed as a cure accelerator for epoxy resin, it is possible to decrease the viscosity of epoxy resin and also to form a cured resin layer which is excellent in electric properties as well as mechanical properties.
In the manufacture of an insulation coil of electric equipment where high heat resistance is demanded, a varnish of relatively high viscosity is employed with a view to enhance the heat resistance of the insulating layer. However, in order to improve the impregnation property of varnish in relative to an insulating tape, the varnish is generally heated in an actual impregnation step so as to lower the viscosity of the varnish. However, once a varnish is heated in this manner, it is impossible to avoid the problem that the storage life of the varnish would be shortened.
On the other hand, in the case of the aforementioned epoxy/acid anhydride-curing type varnish, although the pot life thereof is relatively long because of its low viscosity, a relatively high temperature and a long time are required for the curing of the varnish. Therefore, a cure accelerator is generally incorporated into the varnish. However, when a cure accelerator is directly incorporated into the varnish, the viscosity of the varnish is caused to increase, thus leading to the shortening of storage life as in the case of a varnish of high viscosity.
In the process of impregnating the insulating layer of electric coil with a varnish, a coil is dipped into the varnish filled in a tank for a predetermined period of time, after which another coil is dipped likewise into the varnish, i.e. the same varnish is repeatedly used in this dipping operation. Therefore, the impregnation varnish is desired to be long in storage life, and hence a search for a latent cure accelerator for the impregnation varnish, which does not give bad influences to the storage life of the varnish is now extensively studied.
There are known various kinds of latent cure accelerator, such for example as a quaternary phosphonium compound, an imidazole compound, boron tetrafluoride amine compound, an adduct of tertiary amine with epoxy, a tetraphenyl boron complex and a metal acetylacetonate.
There is also proposed as a means for prolonging the storage life of varnish to microcapsulate a cure accelerator before it is dispersed in a varnish. In this case, the varnish containing the microcapsulated cure accelerator is heated at a temperature higher than a predetermined temperature thereby to melt the capsule, thus allowing the cure accelerator to elute into the varnish to promote the curing reaction of the varnish. However, the aforementioned method is accompanied with the following problems.
Since an impregnation varnish is to be penetrated into an insulating layer of high density, the cure accelerator is required to be completely dissolved into the varnish at an impregnation temperature. However, since the microcapsulated cure accelerator is granulated to have a predetermined particle diameter, it may not penetrate sufficiently into the interior of the insulating layer if the insulating layer is relatively large in thickness. In that case, the interior of the insulating layer may not be sufficiently cured due to an insufficiency of cure accelerator at the occasion of heat-curing. In the worst case, the resin in the varnish may be foamed, thus producing a cured product which is extremely poor in electric properties.
As mentioned above, since the storage life of varnish is shortened or the properties of cured product is deteriorated if a cure accelerator is directly incorporated into an impregnation varnish, a method of incorporating a cure accelerator into the insulating layer in advance instead of varnish has been proposed. Specifically, a method of incorporating a cure accelerator in a binder for an insulating tape, or a method of impregnating a solution of cure accelerator into an insulating tape which has been wound in advance around a coil conductor has been proposed. According to these methods, the insulating tape containing a cure accelerator is dried and then impregnated with a varnish and heat-cured.
However, there is still problems in the curing of the insulating layer impregnated in advance with a cure accelerator in that the varnish in the space (the varnish not containing the cure accelerator) between the insulating layers is caused to cure later than the curing of the portion of insulating layer (insulating base material layer) containing the cure accelerator. As a result, it is difficult to form a uniform cured insulating layer, and at the same time, the resin on the outer surface of the coil may not be satisfactorily cured.
Moreover, there is also a problem that during the process of curing the resin penetrated into the insulating base material layer by heat-treating it in a thermostatic chamber after the insulating base material layer of the coil is impregnated with a resin in an impregnation tank, the resin penetrated into the insulating base material layer may flow out before the resin is cured.
Since a thermosetting resin to be employed as an impregnation resin such as epoxy resin has a property that the viscosity thereof is temporarily lowered due to an increase in temperature thereof at the occasion of heat-curing, the run-out of the varnish in a degree during the aforementioned process of heat-curing cannot be avoided. However, if the resin is allowed to run out of the insulating base material layer, it becomes difficult to form a dense insulating layer. Additionally, if the resin is allowed to run out of the insulating base material layer, voids may be generated in the interior of the insulating layer, so that the electric properties such as corona discharge or the heat dissipation property of the coil would be extremely deteriorated.
Meantime, in view of enhancing the productivity of electric equipment, the impregnation resin is required to be not only excellent in impregnation property and in short-time curability, but also capable of forming an excellent insulating layer irrespective of the kinds of a binder-backed insulating tape.
Because of this, there has been proposed a method of incorporating a powdery latent cure accelerator not only into an insulating layer but also into an impregnation varnish in the manufacture of an insulation coil. However, even with this method, it is accompanied with the problem that the dispersion stability of cure accelerator in the varnish is poor, thus generating a sedimentation during a long period of storage.
If a cure accelerator is added in advance to a binder for the insulating layer, a solution containing the binder resin and the cure accelerator for the manufacture of an insulating tape would be gelled during a repeated use of several times. Furthermore, when an insulating tape is manufactured in this manner, the reaction of the binder would be gradually proceeded during a long period of storage, whereby the tape would become hard, thus making it difficult to wind the tape round the coil.
If an insulation coil is manufactured using such an insulating tape of poor winding workability, not only the external appearance of the resultant coil would be damaged, but also the electric and mechanical properties of the insulating layer would be deteriorated, thus deteriorating the reliability of the insulation coil. Therefore, the insulating tape prepared according to the aforementioned method is required to be stored in a low temperature atmosphere or otherwise required to be employed immediately after the manufacture thereof.
As explained above, the resin composition to be employed as an impregnation varnish for an insulation coil is required to be provided with various properties. However, an impregnation resin composition having such desired various properties as mentioned above is not available up to date.